1. Field of the Invention
The present invention relates to an objective lens, an optical head, an optical system, and a method of designing an objective lens which can be used for an optical disk apparatus capable of recording/playback on a plurality of types of optical disks having different protective layer thickness and recording density.
2. Description of Related Art
There are a plurality of standards for optical disks, with a difference in thickness of a protective layer, i.e. a transparent substrate to cover a recording surface, and in recording density. For example, the thickness of a protective layer of CD (Compact Disk) and CD-R (CD Recordable) having relatively low recording density is 1.2 mm, and the thickness of a protective layer of DVD (Digital Versatile Disk) having relatively high recording density is 0.60 mm, which is half the thickness of CD and CD-R.
The recording and playback of DVD with high recording density require the use of laser light having a short wavelength of about 635 to 660 nm to obtain a small beam spot diameter. On the other hand, the recording and playback of CD-R require the use of laser light having a long wavelength of about 780 nm because of its reflection property.
To allow the use of these different types of optical disks, an optical disk apparatus is preferably compatible with optical disks of any of these standards. In order to enable the use of both DVD and CD-R, an optical disk apparatus needs to have at least two lasers which emit laser light having a wavelength in accordance with each optical disk. Further, in order to downsize an optical disk apparatus, an optical system of an optical head needs to be as compact as possible. It is thus preferred to use a single optical device such as an objective lens for both of two wavelengths and employ a module in which two lasers are integrated into one package as a light source.
Japanese Patent No. 3518684 discloses an optical disk apparatus which enables the use of a single lens as an objective lens for any type of optical disks.
However, in the case of using a module in which two lasers are integrated into one package, light emitting points from the two lasers are arranged in a line along the direction perpendicular to the optical axis of an objective lens, which causes a certain difference to occur in angle of incidence on the objective lens between the laser light beams with two different wavelengths. Accordingly, it is impossible for the incidence angles of laser light beams with two different wavelengths to be both 0, with which wavefront aberration is minimized. It is therefore necessary for the incidence angles of laser light beams with two different types of optical disks to set with the wavefront aberration in an allowable range.
The present invention has been accomplished to overcome the above drawbacks, and an object of the present invention is to provide an objective lens, an optical head, an optical system, and a design method for an objective lens capable of focusing laser beams on an information recording surface while minimizing aberration for each of a plurality of types of optical disks with a different thickness of transparent substrates.